Image display systems contained in helmets or goggle-like frames are known for use in virtual reality systems, where they provide a focused image to the wearer. Such head mounted image display systems typically require long optical paths (upwards of 1.5 inches) between the image source and the imaging optics. The long optical pathway required typically results in a bulky, non-user friendly helmet-like apparatus.
For a image display system worn directly in front of the eye, and assuming a flat image display panel having a display area of about 12 millimeter (mm).times.16 mm, magnification and image location constrain the focal length of any single element "loupe-type" optical system (that is, a system using a simple eyepiece lens). The focal length of the loupe is constrained to approximately 32-33 mm and its distance to the image display panel approximately 30-31 mm. A lower bound on the size of the loupe is also set, at 10 mm.times.13 mm, but this must be increased somewhat to accommodate eye movement. Thus a simple loupe display placed approximately 25 mm from the eye (approximately the distance to the outside surface of an eyeglass lens), and providing an image equivalent to a 13 inch diagonal CRT screen viewed at a distance of about 20 inches, can be no smaller than about 12 mm.times.16 mm and 31 mm in length. These dimensions do not account for the thickness of the components themselves or any housing or packaging. The length and weight of such a system is typically awkward and may result in poor economics in use.
Options for shortening the display system include using a multi-element lens system or folding the optical path. The cost of multi-element lens systems is, however, prohibitive. U.S. Pat. No. 4,859,031 folds the optical path, but decreases light efficiency and is relatively expensive to manufacture. U.S. Pat. No. 5,151,722 provides an eyeglass-like display in which the image source is placed in the sidepieces of the frame and the optical path is folded.